I was on Carnival Freedom last summer and found straightners no problem. They heated up at the same speed as in the UK. So am guessing all Carnival ships similar.

I've just come back from the carbival glory, and whilst my GHD hair straighteners heated up in 3 seconds just as in the UK (they are I beleive, dual voltage!), my hairdryer, which is a powerful one as I have very thick unruly locks, became as pathetic as the on board hairdryer provided in the cabin. So dont waste the precious weight allowance by taking your hairdryer, it's just not worth it!.....